Vacuum diffusion pump



R. A. DENTON VACUUM DIFFUSION PUMP Filed March 2, 1955 May 12,- 1959 INVENToR. Richard .Denznz "d Jtvowzg/ Y United States Patent VACUUM DIFFUSION PUMP Richard A. Denton, Haddonlield, NJ., assignor to The lJew York Air Brake Company, a corporation of New v ersey Application March 2, 1953, Serial No. 339,614 5 Claims. (Cl. 2304-101) 'I'his invention is a multi-stage, vacuum diffusion pump and more particularly a vapor condensation pump that fractionates the pumping liquid and directs its` least volatile fraction to the nozzle ring nearest the pumps intake end and its most volatile fraction to the nozzle ring farthest removed from that end, `and at the same time provides an adequate supply of vapor to `the individual jets and at effective pressure.

Due to their 'lower vapor pressures, the substitution of organic liquids for mercury as the pumping fluid in rvapor condensation pumps made it possible to attain lower pressures and to pump gases at a much faster rate, and for most purposes simplified the problem of trapping vapors that back-stream from the pump.

Nevertheless, the use of organic liquids has several significant disadvantages; among them:

(a) the partial decomposition of the organic liquids by heat, as in thermal cracking, and -in some cases by reaction with the gases and vapors being evacuated, introduces into these liquids contaminating products having higher vapor pressure, and with known undesirable reduction in otherwise attainable vacuum;

(b) this decomposition under heat often is limited t cracking the organic liquids to others of lower molecular weight and does not proceed as far as to complete thermal decomposition (which leaves behind merely carbon) with consequent elimination of the undesirable breakdown products; and

(c) nally while most `of the random oil vapor migrating away from the intake of an oil diffusion pump is easily trapped (by condensation) at temperatures much [higher than those that must be used with mercury, yet it is extremely difficult to trap every bit of oil vapor.

For many applications in producing high vacuum with oil diffusion pumps, the foregoing disadvantages have been overcome at least partially by the fractionating pump, for example, as in the multi-jet, vertical, oil dif- :fusion pump. In it, as the oil in the bottom of the pump is heated to boiling by the heater, the resulting vapor is impelled up through the chimney and is ejected from it and down through the jets at the several elevations to condense on the inside of the cooled enclosing casing of the pump, along which it flows down to return to the boiler. The pumping action is provided by the action of the jets in compressing any gas or vapor molecules or both entrained in the jet streams and ejecting them down toward the bottom of the pump where they are sucked out through the exhaust arm by the mechanyical fore pump. 'Ihis jet action removes the gas molecules dilusing into the ejector vapor stream and the depletion in the vicinity of the stream causes more rapid diffusion of the gaseous molecules from the more remote zones. This results in a continuous pumping action making possible a high degree of evacuation.

Evidently, the ultimate vacuum attainable by such a pump must bear a direct relationship to the vapor pres-v sure of the pumping iluid at the lowest temperature-existing between the intake of the pump and the zones or ICC systems to be evacuated. Thus, any deviation from purity of the pump fluid, Whether by contamination or decomposition, if it results in the presence of higher lvapor pressure substances, will reduce the pumps efciency. Such decrease in purity will not -only limit the ultimate vacuum attainable, but also seriously reduce the pumping speed at the lower pressures. This latter difllculty usually is most serious at pressures below ten to the minus four millimeters of mercury.

Attempts have been made to overcome particularly the' disadvantages caused by the presence of impurities in the pumping iluids, in the form of substances having higher vapor pressure than the pumping uid, whether originating from decomposition or merely as contaminants. These attempts are best exemplied by the fractionating pump covered by U.S. Patent No. 2,112,037 to Louis Malter.

While the Malter fractionating pump and modications of it provided advantages over the simple vertical, multijet pump, nevertheless several shortcomings and disadvantages were observed and experienced even with these fractionating pumps. For example:

(a) in them the fractionation, or division of the pumping fluid into a number of vapor streams having dilferent vapor pressures, is achieved merely by selective boiling oif from individual annular areas of a relatively deep vpool of liquid at the bottom of the pump. Such a pool can vary in depth from one-half to three quarters of an inch in the small pumps, for example, one of four inches pumping chamber diameter and smaller, to two inches and even deeper in the larger pumps;

(b) then the partition of the boiler into several concentric annuli makes it diicult to maintain the proper pressure and adequacy of supply of vapor to the individual jets;

(c) when such a pump stops operating, all of the different fractions of liquid mix together in such a way that when the pump is re-started, it must be operated for a considerable period before the pumping fluid has cycled sufficiently tov establish once more the optimum pumping conditions attainable with the particular invention.

In the pump of the invention, a plurality of coaxial cylinders or tubes, spaced apart radially from one another, are enclosed within and spaced away from, and are co-axial with, the vertical pumping chamber, thus forming a series of coaxial pumping vapor conduits which together constitute the chimney. At its upper end, each of these conduits terminates in an ejector jet directed downward into the pumping zone (defined below).

between the outermost of these and the inner surface of the' pumping chamber is conveniently designated as the pumping zone or area. ferred to as the condensate collection area.

The bottom of the condensate collection area (con-f stituting the lower end of the pumping zone) is closed off from direct access to the interior of the boiler. This can be `done in several ways. A convenient closure is made lby bringing the lower end of the pumping cylinderv andthe outermost vapor conduit together, for example,

,I by providing the lower end of that conduit with a\ down-v ward and outward'flare Vsuthc'ieit to close the interval,"

For, most operations three of such conduits suice. The spaceV Its lower end is sometimes re i from that closure these extends downward into the interior of the boiler a tubular, descending condensate trickler, flume ory cascade toa level nearthat oithe'vbttom end of the inner conduit. Several diierent modilieations offthis condensate trickler are possible, a

Accordingly, all of the descending .condensate .en .route t9 .return to tbe boiler is compelled to .run down the in.-

ner surface of the tubular condensate ttickler .in the. forni of .n relatively thin iilin- At the sante -tirne vapors rising .from .the surface of .the boiling pool of liquid and passing the outer surface of the condensate tricller are restrained .from entering the vapor conduits and reflux in thntrart .of .the boiler- AS a result, the. descending condensate receives heat by direct contact with vapors rising .from the pool of boiling liquid .on their wey tothe vapor conduits and also receives heat conducted through the .wall of the trickler from vapors surrounding the trickler and consequently not in the flow path to the vapor conduits.

.-.Since separation of the dilerent vapor-pressure cornponents of a liquid mixture can be much more easily and more eectively accomplished by evaporation from a thin film, the conditions just described approximate the ideal -for the fractional distillation of the different vapor-pressure constituents of the pumping liquid. Since the most volatile constituents are distilled ott rst and thus abound a-.t the highest level, the outermost or rst pumping .vapor conduit has its lower (or entrance) end higher than that of the other conduits, and these constituents enter it. Accordingly, Iit is the shortest of the conduits and its ejector .jet is the lowest of the jets blowing into the pumping zone. The next volatile constituents abound at a lower level and enter the next inner or second vapor conduit whose entrance end is below that of the outermost orfrst conduit. The second vapor conduithas a height greater than that of the rst so that the jets at Vthe top of vthe second conduit are directed into .the pumping chamber at a level above that of the irst mentioned jets. And so on similarly with the next less `volatile -constituents, .depending on the number of vapor conduits and corresponding jets in the pump. Then the conduit carrying the topmost jet receives the vapors rising directly from the boiling pool 4where the pumping liquid has been rid of its more volatile constituents.V

As to its more specific details, the invention is illustrated by, but not restricted to, the embodiments discussed in thefollowing description in relation to the accompanying drawings, wherein:

vFigure 'l is a vertically transverse section diametrically through the axis of the Vchimney and also of the exhaust or fore) pump arm, and omitting the usual capv or top plate that -fits over, and closes oi, the top of the pumping chamber and has the inlet port through which the interior of that chamber communicates with the space or body-to be evacuated;

-Each of Figures 2 and 3 is a Vsimilar section (replacing that portion of Figure l below thelevel of the bottom of the exhaust pump arm) through another boiler showing in.V each ligure a respectively dierent modification o t' the structureenclosed within the vapor spac'eof'the fhoiler, ffgr distributing the descending condensate'in the form oa descending iilm exposed to frr'ac'tioningrinteraction vnorsrising directlyfrom .the boiling Pool of running liiiiiid and for indirect heet yercb'fuige 'with `other Portions oflltlieyinorsretluuiug iudieboileneud Figure 4 is the cap or top plate to be bolted on'to the top of the puming chamber. YWInthe drawings, the pumping chamber 10 is closed at its upper end by the cap plate 11 secured vacuum tightly to the outward extending annular flange 12 by locking the series of bolts 13 and nuts 14 in the corresponding bolt holes 15. By way of the upward extending inlet port 16, the interior of pumping chamber 10 communicates with the space to be evacuated,

Pumping chamber 10 rises vertically above, and Ycoaxially with,Vv the boiler 18 and has its lower end19 protruding part way through, and terminating below,the vacuum tightly secured closure cap 21 of the boiler 18. Through the exhaust (or fore) arm .2.2, extending outward from the lower end of chamber 10 and vshortly above cap 21 of the boiler, the gases evacuated from the space ibeing evacuated by the pump are withdrawn by the rough pump (not shown).

The lowermost part of the boiler encloses' a continuous, undivided Vlitiuid-Since which sustains .the Pool 23 .et pntnping liquid- Supported, and likewise powered, from either one side (es in Fig l) .o r the bottom (es in Biss. 2 .und 3), of boiler 1.8, a suitablev immersion 'heater ,2.4

t entends into, and, for best effect is completely submerged in, the pumping liquid 23- Around the bottom end of pump chamber 1,() thereis secured an annular, supporting and suspending flange 2 (Fig. l) having a rim extending inward. From'the lower end ofl chamber 10, there depends co-axiallywith said chamber a cylindrical skirt to serve as a condensate. trickler, ume or cascade, down which the return con.-` densate trickles. Three dilerent modications of the .condensate trickler are shown in the .drawings andas a whole lare designated respectively 27 (Fig. 1), 127 (Eig. 2) and 227 (Fig. 3). These are described in detail separately hereinafter.

Enclosed within pump chamber 10 and co-axial withit are three pumping vapor conduits, of which the innerf; most is designated as inner conduit 28, the outermost one as vapor conduit 31, and the intermediate one as intermediate conduit 30. While the modifications included in the drawings have only three vapor conduits, more may be used, or even only two, depending Aon' Vthe particular `job and performance for which the individual pump is .specifically designed. Y

The conduit 28, extends upward higher than the others and, capped with its conicalcap 32, reaches near to the top of pumping chamber 1 0. Spaced apart around the upper end of conduit 28 are a plurality of vapor jet.ori

ces -33 opening under and facing the underside of the downward and outward depending portion of cap 32 which ends in a diameter `greater thanv that of conduit 2 8, so that the jets 34 will pass clear of this conduit.

The outermost conduit 31 has at its lower end an outward extending llare 36 by the lower end of which it lis supported from the top of condensate cascade 27. The latter extends downward toward the p ool of pumping liguid with its lower end as near as practicable toit providing a sufficient gap above the surface of the pool.'

not only to avoid being reached by lthe boiling liigujd but also to compel a large part of the vapors escapingv from it to rise and reflux around the outside surface of cascade 27. The same applies to the distance to cascades 127 and 227 extend downward.

Pdssiblyslightly incre than halfway down its. longue).`

to-conical fractionating bell 40 extends upward .to an in-..

ward vturned shoulder 41 at a height possibly half that of cascade 27 and ending inY a short nipple 42 extending` vftlcaly abQV? shoulder 41. fthe lower end pf @im il 28 yfits inliquid tight engagement around nipple 42 and rests on shoulder 41. 4

A similar frusto-conical bell 43, having its lowermost portion in liquid tight engagement with the inner surface of cascade 27 just above shoulder 37, extends upward fromA it to an inward turned shoulder 4S, at a height about equal with the lower end of iiare 36, and ending in a short nipple 46 extending vertically above shoulder 45. '..Outermost conduit 31 extends to a height somewhat less than half of the height of chamber 10. lIntermediate conduit :30, with its lower end in liquid-tight engagement with nipple 46 'and resting on shoulder 45, extends to a` height somewhat less than half of the distance between the tops of chamber and conduit 31. Spaced around theiupper end of intermediate conduit 30 is a plurality oflvapor jetorifices 47 opening under and facing the under side `of the downward and outward depending skirt. '11n-latter has at its lower end a greater diameter than thatfof conduit 30 and thereby provides the jet 49 ejecting-downward from this conduit.

.Similarly, spaced around the upper end of outer con-v `uit 31 .are vapor jet orices 51 opening under and facing the under side of the aring skirt 52.

A large portion of the vapors rising from the pool 23 ofpumping liquid heated by the immersion heater 24 flows upward along bell 40 into inner conduit 28. Also a portion of the vapors rising from pool 23 passes through` the yapertures 59 in bell 40 and flows upward intol and through intermediate conduit 30, and mingles with vapors that were fractionated out of the lm of returning condensate descending along the inside of condensate flume 27. Finally, a portion of the vapors in bell 43 passesthrough. the apertures 58 in it and flows upward into and through outer conduit 31 and similarly mingles with more volatile vapors fractionated out of the falling filmr of condensate. f

1 The separate streams of vapors continuously owing upward `through each of the three vapor conduits are thus likewise regularly `ejected through the jets 34, 49 and 53 into the pumping zone. There they entrain the'molecules ofthe gas that diffused into the pumping zone from the system ,being evacuated. y.,Helicalcooling coil 55 is wrapped in close contact aroundthe outside of chamber 10. Cooling liquid from fat-,convenient .source (not shown) flowing through coil 55, at a suitable rate maintains the pumping chamber sufijciently cool to `condense the pumping vapors ejected from the jets 34, 49 and 53.

The condensate returnv orifices 56 are made vsmall y enough in size and number for the condensed vapors to accumulate in the condensate collection area 57 to ay height, below the bottom of the exhaust arm 22, great enough for the layer of accumulated condensate to serve as' aliquid seal against the escape of .vapors from conduit llthrough the orifices 56. The,l hot vapors from the boiling pool 23 of pumping liquid reiiuxing around the lower end 19 of chamber 10 transmit heat through it to this liquid seal layer of condensate in collection area 57. Thereby the condensate that regularly passes from the liquidseallthrough orificesv 56 not only is pre-heated initially before entering these orifices but also in being so pre-heated has expelled from it the dissolved gases that were entrained by the vapors ejected from the jets.

Just as the thus de-gassed condensate returns through the orifices 56 to the fractionation Ysystem of the modificationl shown in Figure 1, it returns in the samev way in themodification illustrated in Figures 2 and 3. In both of these the separate streams of vapor owing through the conduits 28, 30 and 31 passed through the same series of operations as described in relation to Figure 1 before returning as condensate through the orifices 56.

In the modication shown in Figure 2, the condensate returning from orifices 56 flows down in form of a descending film over the inner surfaces of the bellows 128 assaase of the condensate trickler or flume 127. Just as with the flume 27 of Figure 1, vapors from the boiling pool 23 of pumping liquid reflux about the' outer side of the flume 127. Then similarly by indirect heat exchange these reiiuxing vapors transmit heat through the bellows to the condensate descending as a film on the other (or inner)y side of them." Accordingly, the most volatile constituents of the condensate (i.e. those having the highest vapor pressure) are fractionated out in about the upper half of the'bellows and rise into conduit 31. Then the fraction with the intermediate vapor pressure is fractionated out in the lower part of the bellows and ascends into conduit 30. lust as in the earlier description this fractionation using the heat absorbed by indirect heat exchanged from the reliuxing vapors is assisted also by the direct interchange with the vapors rising from the boiling liquid and entering the zone embraced within the bellows. Then conduit 28, extending lower than the others receives practically only the rather pure, highest boiling fraction constituting for the most part the boiling pool of liquid.

' The operation in relation'to the modification of Figure 3 is essentially the same` as for that of Figure 2. The only difference is that in place of a bellows as the condensate flume, the condensate annulus 227 consists of a vertical series of consecutively superimposed annular pockets each open on the inner side; i.e., toward the vertical axis of the annulus 227. With this series of pockets the essential difference overthe operation described in relation to Figure 2 is that instead of merely fractionating the film of condensate descending from the orifice 56, a shallow bath of condensate remains in each pocket.

This condition'vprovides an additional advantageous feature in that the condensate reaching these pockets is in its -most contaminated state, because it then contains the various constituents having the higher vapor pressures (i.e. the more volatile ones). Accordingly, when the pumpV is stopped, thesepockets Will still contain the poorest parts of lthe pumping liquid so that they do not mingle with,v the -mai-n pool ou the bottom of the boiler, which then contains the relatively highly purified highest boiling constituent part of the pumping liquid. Thus, whenthe pump is re-started, vapors of this highest boiling liquid can flow directly into the conduit 28 leading to the jetv at thehighest elevation. In addition, the period for any furtherpuriiication will be considerably shorter. f In all of the modifications, vapor rises into all of the jet conduits' from the same uninterrupted and undivided boiling -poolof liquid at the bottom of the boiler. Thus,

- at worst there are only minor differences in pressure at the Ventrances to the several jets. Accordingly, it is impossible to build up undesirably large pressure differences -between the jets. y

The `bottom of the condensate fiume, as in Figure 1,

l or the bottom'ofthe inner conduit, as in the modification of Figures 2 and 3, can extendto from one-half inch tov one inch and more, above the top surface of the pool of liquid on the bottom of the boiler.

' While the uninterrupted and undivided :pool of pumping 'liquid at ,the bottom `of the boiler permits using the more rapid and very efficient immersion heater, Isome other form of heating now available, if desired, can be used in connection with the other advantageous features of the invention. Likewise, any of the available-or applicable pumping liuids can be used with the pumps o f4 the invention, such vas the hydrocarbon oils, the halogenated. hydrocarbons, the various dialkyl phthalates, or

other esters, or the types of silicone oils used, or others. There is no restriction to the specific proportions or relative positioning of any of the parts of the pumps that are variable in specific proportions or relative positioning. Also there is no restriction to only one jet on each pumping conduit as more than one can be used so long as the fractionation feature is retained. 'I'hen also any of the practicable jets can lbe used, as well as either a plurality of single jets arounda circle at the jet elevation iii 'single iet built the 4eiieiiliir ferie.

, ,flfil'revvise otherl foin-s of practieable condensate tieners er iliiiis 'can be used. Per exeiiiple, the' oiie: Figurey 1, can have included Vin either its upper or lower half, or both, one or'more shoulders, q'uite similar tje shoulder 37, iieifer supperiiiig einer parte', but rather to `iiicrease'tll'e path, or slov'v down the rate, vof descent of the descending condensate. Alternatively, there could be ,used such a shoulder in 'the forni of a helix. Sintiieily, nie Ylielldw's in Figure 2, er nie pockets er Figure 3, could be made in the form of a helix. However, converting the superimposed pockets of Figure 3 to a helix form eliminates the added aclvarita'geous feature of serving to keep out of the niain pool the poorest fractions of pumping liquid, when the pump is shut down, as has been described.

Moreover, many changes Vcan be rnade i-n the specific shape of any of the individual parts, or more than one cany be combined in one, tousirriplify steps, for example, individual spinnings, to be used `in constructing the pump.

Sofr'rewell recognized parts ordinarily used may not be. shown, in the drawings, ifor example, a plug in a lower of the boiler to provide an aperture through which the pumping liquid can lbe withdrawn. However, Aone such form shown in Figure 2 by the outlet 60 audits screwcap 6 1 a-nd compressible inert plug 62. Along this line, none of the drawings :shows an aperture and closure in an upper part ofthe boilel 'to lpmit adding pumping iquid. n

A'In general, the construction of. Vthe pli-iup is convenient f ,or assembly and for cleaning. Thus, as seen vin the fieation of Figure l, the entire chimney and the accom-A panyingfraotionation Structure can be lifted through the top as asingle completely assembled unit. Thus', while the various features of the invention have been described in detail in relation to certain` 'speciii'c embodiments of it, itis understood that various modiiieations and substitutions be made within the bounds the appended claims vwhich are intended `also to cover quivalents of thespecific embodiments.

fWhat isi':laimf':"d.is:y j 'vacuum pump of the diffusion type comprising in nation, a chamber having an inlet Conli'ectloi and a connection Ifor a fore-pump, and including. a condensing surface along which for-ming condensate tenuate new by 'greviryya boilerfbeiieiitliseidpmiiping, ehiiilier and pe'rtiallynued with 'volatile liquid, se that the boile encloSs lva liquid-filled space, audabove the level Vofsaid liquid also a vapor-space; ineans lfor heati'ii'g'sid liquid; llovv restifictor lstructure affording limited dife'ct communication between said vapor-space and said pumping chamber in which restrictor 'structure condensate leaving said condenser surface `collects; and substantially inhibits direct ow of vapor fro'r said vapoispace to "je pumping chatiibr,"whle permitting return new` of condensate; a plurality of jet units i'r'l the pumping chainberarranged to direct vapor toward the for'epur'np 'conriecfin and inp'roxniity to fsid jeend'eusiiig surface, seid jet `units Abeing located at diterent respective heights; segare'` vapor-now 'connections from respective intakes loeated in th'e vapor-space fat different respective velevatio syto corresponding *jet units, in which intakes of succssivelv greater elevation lead to'jet units of y"successivel'y lower heights; and "a tfrickl'e'r structure within said vaporspace and in' the path of condensate leaving said restrictor structure, for effecting active heat exchange between said` condensate and vapor ovving to said intakes.

f2. The combination defined in claim 1 in which the txickler :structure is-an--annularly corrugated 'open ended tubular sleeve in which the annular corrugations are spaced in successive substantially horizontalplanes 'and serve to lengthen the flow path for liquid and reducelthe gradient whichinduces'liquid ow.

3,The combination dened in claim 1 in whichjthe' trickler' is an annularly corrugated vopen ended sujbsta separates a vapor space trom `a liquid-filled space',` the boiler being at a lower level than said pumping chamber; a chimney comprising tubes of graduated diarne substantially coaxial with one another and with 'said chamber, the outermost tube being-sealed at itsilowe'r end to the enclosing Wall of the tubular pumping chain-f ber, but formed with at least'one 'liquideflovverestiietive aperture Which aiordsa 'condensatedrain into'said boiler, the lower ends of all said tubesbeing Within andopen'to the vapor space of the boiler, the innermost'tube'and the intervals between successive tubes each leading to a corresponding jet unit directed into said pumping 'chamber toward said fore-pump "connection and in proxir'nityto sai-d condensing surface, the'innernl'ost tube having V"che lowest entrance from the boilera'nd thel highest jet uit,

' the` outermost inte'rtube interval having thejhigheist'enl trance from the boiler andthe lowest jet unit, intervening ite'rtube intervals having respective heights spac'ed'in progressive 'sequence between the heights just s'ta'tedgjan annular trickler 'structure Withinsaidlvapor space,""en f circling -the entrances to -said chimney tubes end l'ocatd in the path of condensate discharging fromsaidflovvjrcf strictive drain, said trickler serving to intensify heat exi change lbetweenV sai'd condensate and `vz'iygrorgenerated in the boiler; 'and means for heating liquid in the boilet 5. A multi-stage vacuurnrpump comprising in colrlbil'a-LY tion, ja pumping chamber having an inlet near oneend for elfecting connection to a 'space to be''e`i l a `:ug`ated,t an outlet near its other end and a condensing 'surfacebetvvee s'ai'dinlet andy outlet; means for sustaining a'bathof pumping liquid and enclosing a vapor space "z abo've)the surface' of said bath; heating' meansfor vap'orizing's'id bath; Sa chimney structure for receiving vapor c'iven4 from said bath and delivering it into said pufr'npingj eha`1ii--V ber in jets directed against said 4condensing surfacegsaid chimney being sub-divided into a pluralityjof passages,

which, respectively lead from a 'series of increasing above the liquid bath and respectively terminate/i' jets aforesaid at 'aseries of diminishing heights 'Within the pumping chamber; land trickler ymeans Within said vapor space and in the path of condensate flowing from sai'd condensing surface' forrece'iving Vsaid 'condensatejand passing it in heat exchange relation with vapory flowing to the inlets of 'the 'charnn'ey passages, Awhereby heat exf change Vfavors Vfractionation and the delivery 'asvporfof the l'eastv'olatille fraction to the jet closest tol the inlet o'f said pumping chamber and so on through the References Cited in the -le of this patent V UNITED STATES PATENTS 

